Blood disorder (see hemoglobinopathy) seen mainly in persons of Sub-Saharan African ancestry and their descendants and in those from the Middle East, the Mediterranean area, and India. About 1 in 400 blacks worldwide has the disease, caused by inheriting two copies of a recessive gene that makes those with one copy (about 1 in 12 blacks worldwide) resistant to malaria. The gene specifies a variant hemoglobin (hemoglobin S or Hb S) that distorts red blood cells (erythrocytes) into a rigid sickle shape. The cells become clogged in capillaries, damaging or destroying various tissues. Symptoms include chronic anemia, shortness of breath, fever, and episodic crises (severe pain in the abdomen, bones, or muscles). Hydroxyurea treatment triggers production of fetal hemoglobin (Hb F), which does not sickle, greatly lessening severity of crises and increasing life expectancy, previously about 45 years.
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